It depends on the your body, your work & activities and the environment/ climate you live in. Some say 2 liters (arbitrarily). Institute of Medicine recommend 2.7 liters for women & 3.7 liters for men! Well, 60% of human body is water & when that water content goes below certain level, you feel the urge to drink water. Also, when you are sweating, exercising & during hot weather, you feel more thirsty & drink more water. So, your body has built in system for when & how much water you should drink. You don’t need to think about it.